Transparent Electronics for Active Matrix Displays

Thomas Riedl

Institute of Electronic DevicesFAHL 2010, Leipzig, 28.09.2010

Transparent Electronics for Active Matrix Displays– Slide 1

Transparent Electronics for Active Matrix Displays

Thomas Riedl

Lehrstuhl für Elektronische Bauelemente, Bergische Universität Wuppertal

Thomas Riedl



City: WuppertalState: North-Rhine-Westfalia

Pop.: 353.308

City of Wuppertal

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Thomas Riedl



Bergische Universität Wuppertal

since 1863 School of Engineering

1972 Foundation of BU Wuppertal

7 Faculties

14,000 Students

Thomas Riedl



Electrical, Information and Media Engineering

1100 Students

21 Professors

Focus Areas:

→ Automotive Engineering

→ Print & Media Technology

→ Renewable Energy Systems

→ Polymer Electronics

Campus Freudenberg

Thomas Riedl



Institute of Electronic Devices, University of Wuppertal, Germany (T. Riedl)

since : 01.10.2009

former: Advanced Semiconductors Group

Institute of High-Frequency Technology, TU Braunschweig

Institute of Electronic Devices

Thomas Riedl



New Labs - Institute of Electronic Devices

400 m2

220 m2 lab space (clean-room)

Start-up: 2010

Thomas Riedl



Institute of Electronic Devices, University of Wuppertal, Germany

Reliability Analysis

Transparent Oxide-Electronics

OrganicPhotovoltaics

Organic ElectronicsOLEDs

Functional Molecular Systems

Organic Lasers/Amplifiers

Research Topics

Thomas Riedl



→ Transparent Electrodes (OLED, OPV)

→ Thin-Film Encapsulation

→ Oxide TFTs

Metaloxides as Components of AM-Displays

Thomas Riedl





→ Oxide TFTs


Thomas Riedl



OLED Displays

→ no back-lighting

→ large viewing angle

→ low weight

→ flexible

→ cost effective technology

→ low energy consumption

LG 15 inch OLED TV (15EL9500)

Samsung S8300 UltraTOUCH

Thomas Riedl



OLED Lighting

OLEDs: next-generation lighting technology

Thomas Riedl



Transparent Displays

Minority ReportTM and © 2002 Twentieth Century Fox and Dreamworks, LLC. Transparent OLED AVATAR TM and © 2009 Twentieth Century Fox

Thomas Riedl



Transparent Organic Light Emitting Layers

taylored organic layers: + absorption in the UV (< 400 nm) + emission in the visible

→ large „Stokes shift“

300 350 400 450 500 550 600 650 700

5.0x104

1.0x105

1.5x105

0.0

0.2

0.4

0.6

0.8

1.0 Absorption Emission

abso

rptio

n co

effic

ient

, cm

-1

wavelength, nm

PL in

tens

ity, a

.u.

guest-host systems

TCTA

Ir(ppy)3

Thomas Riedl



Hardware

Metall-Cathode

Organic Semiconductor

Transparent Anode e.g. ITO

Glass-Substrate Light extraction

⇒ entirely transparent OLEDs

Transparent Cathode (TCO) Light extrcation

Thomas Riedl



Transparent Conducting Oxides on Top of Organics

Organic semiconductors are brittle !

→ low processing temperatures (< 80 °C)

→ avoid oxygen + high-energy particles/radiation

Bonding energies:C-C (3.73 eV), C=C (6.21 eV)

Typical by-products : charge carrier trapsnon-radiative defectsbut:

→ transparent oxides contain oxygen→ sputtering, pulsed laser deposition→ particle energies ∼1-100 eV ?→ UV radiation

Thomas Riedl



Transparent OLED Structure

Adv. Mater. 20, 3839 (2008)

→ bullet-proof vest (WO3, MoO3, …)

→ thermal deposition of WO3, MoO3, …

→ WO3, MoO3 transparent, conductive

→ record-efficiency: 30 lm/W, 38 cd/A

Thomas Riedl



WO3 as Sputter Protection Layer - SIMS Analysis

Analysis of particle penetration → secondary ion mass scectroscopy

Sputter particles penetrate WO3 layer only by 40 nm → protection for OLED

Thomas Riedl



MoO3 High-Work-Function Material

Appl. Phys. Lett. 95, 123301 (2009)

→ transition metal oxides: n-type semiconductors

→ no electron blockers

→ high workfunction, very deep valence band

http://www.epio.info/

http://www.epio.info/

Thomas Riedl



Transparent OLEDs

Thomas Riedl



Semi-transparent Organic Cells

Transparent electrodes → semi-transparent OPV

currently ∼ 2.5 % efficiency

lightAppl. Phys. Lett. 94, 243302 (2009)

Thomas Riedl



Vision

Terminal 2G Airport Paris Charles de Gaulle

Thomas Riedl



Indium-Tin-Oxide (ITO) as TCO

+-

→ Indium is scarce and expensive

→ resources last for only 20 years(US geological survey 2006)

Thomas Riedl



Issues with Indium-Tin-Oxide (ITO)

→ identify alternatives

ITO consumption in 2006 and 2030

Quelle: Fraunhofer Gesellschaft 2009: „ Rohstoffe für Zukunftstechnologien“

Thomas Riedl



Al doped ZnO

ZnO is cheap and abundant !

• ZnO:Al2O3 (4 wt%)

• pulsed laser deposition (PLD)

• sputter deposition

„our“ AZO:

→ optimum conductivity: 4000 S/cm (n=6x1020 cm-3; µn=42 cm2/Vs)

c.f. ITO (MERCK) 4700 S/cm

→ high transmissivity in the visible:

@ 550 nm: α=200 cm-1 → T= 98 % (1µm film)→ Rsheet = 4 Ω/sq. Appl. Phys. Lett. 91, 041113 (2007)

Appl. Phys. Lett. 93, 073308 (2008).

Thomas Riedl



ZTO/Ag/ZTO

Zinc-Tin-Oxide (ZTO)

→ high carrier mobility

→ highly transparent > 80 %

→ simple processing technology

sputter-deposition at room temperature

ZTO (30 nm)Ag

ZTO (30 nm)

400 500 600 700 8000

20

40

60

80

100

6nm Ag

trans

miss

ivity

(%)

wavelength (nm)

ZTO/Ag/ZTO

70nm ITO (pure)

100

101

102

103

104

105

0 2 4 6100

101

102

103

104

cond

uctiv

ity (S

/cm

)

Ag thickness

shee

t res

istan

ce (Ω

/sq)

RT ITO

Thomas Riedl



ZTO/Ag/ZTO

Thomas Riedl



Alternative Transparent Electrodes

Metall Oxide

Metall Oxide

charge transfer

Charge transfer → doping of the oxide

Thomas Riedl




TPBi

TPBi:Ir(ppy)3

ZTO 42 nm

ITO

BPhen:Cs2CO3

Ag 10 nm

ZTO 42 nm

MoO3

TCTA

0 1000 2000 30000

10

20

30

40

50

10

20

30

40

50

curre

nt e

fficie

ncy

(cd/

A)

luminance (cd/m²)

powe

r effi

cienc

y (lm

/W)

ηphot = 43 cd/A, ηlum = 30 lm/W @ 1000 cd/m²

Kooperation T. Winkler, TU-BS

Thomas Riedl




>80 % transmission @ 500 nmdefect free!

Kooperation T. Winkler, TU-BS

Thomas Riedl





→ Oxide TFTs


Thomas Riedl



Thin-film Encapsulation for OLEDs

substrate

insulationorg. thin filmsmetal

ITO

Glass lid

Conventional glass-lid encapsulation:

→ not flexible, heavy

→ expensive

→ problematic for transparent OLEDs

⇒ efficient thin-film encapsulation needed

„food packaging“

Thomas Riedl



Temperature aspects

HTL ETL

Tg = 95 °CTg = 170 °C

→ organic materials form glassy films: typically Tg = 50 - 200°C

→ Low-temperature (T < 100 °C) encapsulation process required

Thomas Riedl



ALD: Technique for extremely dense and conformal dielectric layers

highly reactive precursorse.g. for Al2O3 Trimethylaluminum (TMA) and H2O

→ low processing temperatures (< 100°C)

→ promising for barrier layers for organic devices

Atomic Layer Deposition

Thomas Riedl



Calcium as Sensor

Array of Ca Pads

Without encap → 100 nm Calcium completely oxidized (90 s in air)

expected: with encap. (WVTR=1x10-6 g/m2 d) → 700 days

Thomas Riedl



Corrosion of Al2O3

100 nm Al2O3

Large area encapsulation (80 °C, 80 % RH)

fresh 48 h 118 h

95 nm Al2O3 + 5nm ZrO2

Thomas Riedl



Nano-laminates as Encapsulation Layers

SubstrateOLED

cyclic deposition of Al2O3 and ZrO2

20 cycles Al2O3 (2 nm)20 cycles ZrO2 (3.8 nm)

aim:

→ protection against corrosion

→ forced amorphicity (neat ZrO2 polycrystalline)

→ avoid permeation channels

next generation thin film encapsulation → nano-laminates

Adv. Mater. 21, 1845 (2009).Tetrakis(dimethylamido)zirconium(IV)

Precursor for ZrO2 preparation

Thomas Riedl



20 h 60 h 160 h

100 nm Al2O3 + ZrO2 nanolaminate

Statistical Defects

Adv. Mater. 21, 1845 (2009).

Large area encapsulation (70 °C, 70 % RH)

Thomas Riedl



encapsulation permeation rate forwater

(g/m² day)

permeation rate foroxygen

(cm³/m² day)Al2O3 130 nm @ 80 °C 8.8 x 10⁻⁵ 3.9 x 10⁻²

Al2O3 & ZrO2 130 nm @ 80 °C 4.7 x 10⁻⁵ 2.1 x 10⁻²

Test conditions (climate cabinet): 70 °C and 70 % RH

Permeation Rates

Adv. Mater. 21, 1845 (2009).

with Ea = 92 kJ/mol → 5 x10-7 (at RT)

→ Low permeation rates maintained down to 40 nm thin laminates !

Thomas Riedl



OLED Lifetime

w/o encap: 350 h

Al2O3: 8,700 h

ZrO2/Al2O3 NL: 22,000 h

glass lid + getter: 56,000 h

Substantially improved OLED lifetime through ALD nanolaminate

→ close to benchmark of glass lid encap

Starting luminance 1,000 cd/m2, constant-current mode

Appl. Phys. Lett. 94, 233305 (2009).

Thomas Riedl



Underwater OLEDs

ALD barriers allow for wet chemical post-processing:

→ photo-lithography→ printing

Thomas Riedl





→ Oxide TFTs


Thomas Riedl



Transparent Displays

Thomas Riedl



Transparent AMOLED Displays

Ice Touch YP-H1 - Samsung CES 2010

Thomas Riedl



Passive-Matrix OLED (PMOLED) Displays

⇒ pixel brightness >> display brightness

e.g. VGA-Display (640 x 480 pixels) 100 cd/m2

→ required pixel brightness > 48,000 cd/m2

→ reduced efficiency, reduced lifetime

Passive-Matrix Display:

→ row by row addressing

→ ok for small area/low resolutionanode

cathode

solution: active-matrix addressing

Thomas Riedl



Active-Matrix Addressing

Thomas Riedl



BMBF Project:

Grundlagenuntersuchungen zu Transparenten Elektronischen Bauelementen

und Schaltungen

Thomas Riedl



Transparent Semiconductors

ZnO-SnO2 : abundant, non-toxic

wide band gap (> 3eV) → see-through

deposition by sputtering or PLDMinami, MRS BULLETIN/AUGUST 2000

Thomas Riedl



Transparent TFTs on Glass

channel: (ZnO)x (SnO2)1-x (ZTO)

Drain-Source Electrodes: ZnO:Al (AZO)

Gate dielectric: Al2O3 (by atomic layer deposition)

• TEM: ZTO amorphous structure • Transmissivity > 80 %

Bottom-Gate stucture

ZTO

epoxy

gate dielectric

ITO

glass

TEM cross section

TEM images:Dr. Thomas WeimannPeter Hinze

Appl. Phys. Lett. 90, 063502 (2007)

Thomas Riedl



Plasma assisted deposition

-40 -30 -20 -10 0 1010-10

10-9

10-8

10-7

10-6

10-5

10-4

atomarer O

I DS (

A)

UGS (V)

molekularer O

-40 -30 -20 -10 0 10

0.000

0.002

0.004

0.006

0.008

0.010

atomarer Omolekularer O

I ds0.

5 (A0.

5 )Ugs (V)

→ severe hysteresis → defect states

→ elevated off current

→ Threshold shifted towards negative Ugs → elevated carrier density

µsat = 6 cm2/Vs

µsat = 10 cm2/Vs

Oxygen partial pressure: 2x10-4 mbar (PLD)

UthrUthr

Appl. Phys. Lett. (in preparation)

Thomas Riedl



400 500 600 nm 800

0

20

40

60

%

100

(ZnO)x(SnO2)1-x , d=1µm atomar O, molecular O,

Tran

smiss

ion

λ

Optical Properties

• many sub-bandgap states

• shallow donors

→ ZTO carrier density (Hall):

Molekular O : n = 1019 cm-3

Radical O : n = 2×1016 cm-3

radical O molecular O

ZTO-layer thickness: 1 µm

Thomas Riedl



ESCA Analysis

ZTO Layer

photoelectronsX-rays

SiO2 Substrate

O1sZn2p3/2Sn3d5/5

SiO

2

SiO

2

ZTOZTO

Gradient in [Zn]-content → out-diffusion of Zni → defects: excess zinc (Zni)

0 10 20 30

44

45

46

47

48

49

50

51

O c

onc.

(at

%)

x (nm)

atomar Oxygen molecular Oxygen

-5 0 5 10 15 20 25 30

0.92

0.94

0.96

0.98

1.00

1.02

[Zn]

/([Zn

]+[S

n]) (

norm

.)

x (nm)

atomar O molecular O

Sputtering (Ar-Ions)

Thomas Riedl



TTFT transfer-characteristics

Ion/Ioff=106

µFE,SAT = 13 cm2/Vs

(c.f. a-Si: µFE,SAT = 1 cm2/Vs)

Uth = -1..1 V

→ no hysteresis measureable→ low sensitivity towards visible light

TTFT output-characteristics

ZTO-TTFTs on Glass

0 1 2 3 4 5 6

01020304050607080

Ug = -3...5 V

I D (µA

)

Uds (V)

-6 -4 -2 0 2 4 61E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

-2.0x10-3

0.0

2.0x10-3

4.0x10-3

6.0x10-3

8.0x10-3S = 0.45 V/decade

µfe = 13 cm2/Vs

I D (A

)

UG (V)

Vth

I D1/2 (

A1/2 )

phys. stat. sol. (rrl) 1, 175 (2007)

Appl. Phys. Lett. 90, 063502 (2007)Appl. Phys. Lett. 91, 193504 (2007)

Thomas Riedl



TTFTs and Transparent OLEDs

P. Görrn et al., Adv. Mater. 18, 738 (2006) building block of transparent AMOLED displays

Thomas Riedl



ZTO Pixeldriver

C = 12.3 pF

W/L = 100 µm/10 µm

pixel area: 180 × 240 µm2

transmissivity 80 %Solid State Electron. 53, 329 (2009). IEEE/OSA J. Displ. Technol. 5, 810 (2009)

Thomas Riedl



ZTO Pixeldriver

Requirement:

full HD (1,920x1,080 pixels) + 100 Hz refresh

frame-time 10 ms

max. pixel charging time ∼10 µs

Solid State Electron. 53, 329 (2009). IEEE/OSA J. Displ. Technol. 5, 810 (2009)

Thomas Riedl



smart pixel active pixel

stability considerations for analog driving TFT:

drift in the TFT device parameters ⇒ drift in pixel brightness

Transparent Active Pixel

Thomas Riedl



a-Si TFT Instability – Ghost Images

Original image (stress for TFTs) homogeneous background withGhost image

Lee

et a

l. IE

EE

Ele

ctro

n D

evic

e Le

tt. 2

7, 8

30(2

006)

AMOLED Display with Si TFT backplane

Thomas Riedl



TFT Stability

Shift of the threshold voltage after 10 h gate bias stress Ug=10 V

Thomas Riedl



TFT Stability

Depending on the composition, the threshold is ….

• …. increased by trapping of electrons in the dielectric• …. lowered by deep state creation in the ZTO

Thomas Riedl



30 35 40 45 50 55 60 65 70-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

250°C 350°C 450°C

∆V th

/V

Zn:Sn ratio (%)100 1000 10000 100000

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

stress time/s ∆

V th/V

Zn concentration (%)65

333836

46

45

Appl. Phys. Lett. 90, 063502 (2007)

[Zn]:[Sn] compositions for extremely high TFT stability

Shift of the threshold voltage after 10 h gate bias stress Ug=10 V

Our devices with varied [Zn]:[Sn] ratio

Compositional Dependence

Thomas Riedl



1,000 h Lifetime Test

after 1,000 h: ∆Vth = 320 mV for DC current stress 100 × IOLED

• c.f. a-Si TFT ∆Vth = 2 V after 5 h (A. Nathan et al. J.Vac. Sci. Technol. 24 , 875 (2006))

• IGZO TFT ∆Vth = 1.6 V after 10 h stress (SAMSUNG SID 2008)

⇒ ZTO unsurpassed stability for TFTs with amorphous semiconductorphys. stat. sol. (rrl) 1, 175 (2007) Appl. Phys. Lett. 90, 063502 (2007)

-6 -4 -2 0 2 4 6 8 10 1210-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

I ds/A

UG/V

before stress after 1000h

Thomas Riedl



Ambient Temperature

Thomas Riedl



Impact of Illumination – IGZO Case

Thomas Riedl



Illumination – Issues with Zinc Indium Oxide

Thomas Riedl



Illumination – Zinc Tin Oxide

Strong influence of:

→ wavelength

→ annealing temperature

Thomas Riedl



Illumination– Zinc Tin Oxide

Thomas Riedl



Interaction with Oxygen - IGZO

problematic for encapsulationas required in OLED displays

Thomas Riedl



Encapsulation of Oxide TFTs – Zinc Tin Oxide

P. Görrn, T. Riedl et al. J. Phys. Chem. C 113, 11126 (2009).

→ Higher binding energy of O2 in ZTO-

Thomas Riedl




J. Phys. Chem. C, 2009, 113 (25), pp 11126–11130

Thomas Riedl




→ change of threshold voltage ∆Vth < 1V

J. Phys. Chem. C, 2009, 113 (25), pp 11126–11130

novel encapsulation strategy:

→ deposit thin (1nm) encap. first

→ re-expose to oxygen

→ oxygen can re-adsorb at channel

→ apply full encapsulation

→ pre-encap. clamps adsorbed Oxygen

Thomas Riedl



Future Prospects

→ target 3-D Displays

→ resolution > full HD

→ 480 Hz refresh

→ beyond reach for a-Si electronics

→ metal-oxide TFTs key technology

Thomas Riedl



Printed Oxide TFTs

harvest benefits of both worlds:

→ stability/performance of in-organic oxide semiconductors

→ large-area, high-throughput, low-cost printing techniques(formerly domain of polymer electronics)

Thomas Riedl



Printed Oxide TFTs

Thomas Riedl



Printed Oxide TFTs

In2O3 (In chloride) 400 °C

Zn-In-O (Zn chloride, In choride) 600 °C

Zn-Sn-O (Zn acetate ,Tin acetate) 500 °C

Ga-In-O (Ga(NO3)3) 600 °C

Ga-In-Zn-O (ZnAc, Ga(NO3)3, In(NO3)3) 300 °C

In-Sn-O (In chloride, Tin chloride) 250°C

........

Literature reports for liquid processing of metal oxides

Metal-oxide system Precursors used Annealing temp

Thomas Riedl



Printed In-Zn-O TFTs

Precursors:

Indium nitrateZinc acetate

Process-temp.: 450 °C

µFE,SAT > 6 cm2/Vs

Thomas Riedl



Printed Oxide TFTs

stability vs. stress ?

hysteresis ?

Recent data 5 cm2/Vs

Thomas Riedl



Printed Oxide TFTs

Thomas Riedl



Printed Oxide TFTs

ZnNO3,Ga(NO3)3, In(NO3)3 aqueous solution

Metaloxide formation at 95°C

Mobility: 2.3 cm2/Vs

Y.-H. Yang et al.

Thomas Riedl



Summary

Transparent ZnO-SnO2 TFTs 80 % transmissivity, outstanding stability

Transparent OLED technology> 40 cd/A , 80 % transmissivity

ALD Thin film encapsulationWVTR 5×10-7 g/(m2day), lifetime > 20 000 h

Transparent drivers electronics100 Hz, full HD, >4,000 cd/m2

Thomas Riedl



Acknowledgement

Co-workers at :

TU Braunschweig

PTB

Princeton University

Innovation Lab Heidelberg

Funding:

Transparent Electronics for Active Matrix Displays

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